The invention relates to a motor assisted steering apparatus which permits a steering force required to operate a steering wheel or the like to be reduced by utilizing an assistance from a motor, in particular, to an abnormality detector for motor assisted steering apparatus which detects an abnormal operating condition of the motor.
Considering an automobile, for example, when a steering wheel is turned either to the left or right, such motion is transmitted to a steering mechanism to direct the orientation of front wheels either to the left or right. Assuming a constant road condition, the magnitude of a force which will be required to turn the steering wheel will increase as the running speed of the automobile decreases, requiring a maximum force when the automobile is at rest. However, in many instances, a turning motion of the steering wheel through an increased stroke is required when the automobile is at rest or running at low speeds. For this reason, there has been proposed a power assisted steering apparatus which facilitates an operation of a steering wheel by utilizing a power assistance in the steering system. In such apparatus, an auxiliary drive force which depends on the turning force applied to the steering wheel is transmitted to the steering mechanism to assist in the operation of the steering wheel. The power assisted steering apparatus may be either electrical or hydraulic type depending on the kind of means which is used to develop the auxiliary drive force. A majority of apparatus which is currently incorporated in automobiles is of the latter type.
However, a power assisted steering apparatus of hydraulic type requires various components such as a hydraulic pump, control valve, hydraulic cylinder or the like, which must be connected together through hydraulic pipings, resulting in an increased size of the apparatus. In addition, a number of mounting operations are required due to the presence of hydraulic pipings. For this reason, attention is recently being directed to a motor assisted steering apparatus which is simple in construction and rends itself to a reduced size construction.
A basic idea of such motor assisted steering apparatus is disclosed in Japanese Laid-Open Patent Application No. 63,265/1984. In this apparatus, a turning force applied to a steering wheel is detected by a torque sensor, and an electric motor is energized in accordance with the detected magnitude of the torque. The turning force applied to the steering wheel is combined with the drive from the motor before their sum is transmitted to the steering mechanism.
The reliability of an electrical control system is of a great importance in the motor assisted steering apparatus of the kind described. In the event a circuit which drives the motor becomes short-circuited due to oscillations of the automobile or a temperature rise, the wheels may be directed independently from an operation of the steering wheel by the driver. Accordingly, a research and development of motor assisted steering apparatus has been conducted along with means to prevent its malfunctioning.
By way of example, in Japanese Laid-Open Patent Application No. 209,365/1985, an energizing current for the motor which develops the auxiliary drive force is monitored, and the energization of the motor is inhibited whenever the energizing current exceeds a threshold value which is established in accordance with a vehicle speed. It is reasonable that the detection of an abnormal energization of the motor is based upon a comparison against a threshold value which depends on a vehicle speed inasmuch as the driving force which is required to direct wheels through the steering mechanism is closely related to the vehicle speed. However, in the arrangement described, no attention has been paid to the turning force applied to the steering wheel. Specifically, the occurrence of an abnormality is not recognized if the energizing current of the motor is equal to or less than the threshold value which depends upon the vehicle speed. Accordingly, the motor may be energized with an energizing current equal to or less than the threshold value even if the steering wheel is not operated.